In some commonly known electromagnetic clutches having a stationary magnetic core, a rotor and a relatively rotatable armature, an air gap separates the rotor from the armature when the electromagnet is de-energized. The armature is held away from the rotor by means of leaf springs secured to a pulley assembly which, in turn, is keyed to the shaft about which the clutch rotates. A multiple-turn winding (i.e., clutch coil) is disposed between the magnetic core and the rotor and, when energized, produces magnetic flux which threads a path through the magnetic core, the rotor and the air gap to the armature whereby the armature is drawn toward the rotor. By way of this flux coupling, the armature is moved to close the gap and engage the rotor so the two are coupled by friction and one drives the other without slippage. The coupling torque between the rotor and the armature is dependent in part upon the m.m.f. produced by the coil and the magnetic flux threading the interface between the rotor and the armature.
Typically, when full or rated voltage is applied to an initially de-energized clutch coil (i.e., a step voltage), the current rises exponentially due to the inductance of the coil. In a gap-type electromagnetic clutch, at a predetermined level of current the m.m.f. in the magnetic path becomes sufficient to pull the armature into contact with the rotor against the bias of the springs. At the instant of gap closure (i.e., touching of armature to rotor) the coil current and the m.m.f. may have almost reached the rated or maximum values, but the flux is still rising because the reluctance of the entire flux path falls dramatically as the gap narrows and closes. Because torque transmission between a touching rotor and armature is proportional to the flux crossing the interface, if rated voltage is applied at a first instance to the coil, the armature more or less slams into engagement with the rotor at a later second instant with a slight delay determined by coil inductance and mechanical inertia. But at the second instant, torque transmission between the rotor and armature virtually jumps from zero to the rated value.
Such a jump in torque may cause (i) an undesirably sudden loss in speed of the prime mover supplying input power to the clutch, (ii) undue shock or strain on driving on driven components, including belts or chains, or (iii) unpleasant engagement noise and belt screech. In addition, when the rotor and armature are engaged the inertia of the slower moving of the two (and its load) needs to be overcome before the full torque coupling locks the rotor and armature into synchronized rotation. After touching (i.e., initially after gap closure) and while the rotor's or armature's inertia is being overcome, frictional slippage occurs at the rotorarmature interface due to the existence of the maximum or rated magnetic attraction force; this latter condition often results in alternate slips and holds (i.e., chatter noise and undue wear). This alternating slip-hold vibrating engagement or chatter is sometimes evidenced by a loud audible vibration or "screeching" noise generated at the rotor-armature interface.
Some clutches and brakes have been associated with control units which produce a so-called "soft start" action. In these, the average coil current and the average m.m.f. are smoothly increased from zero to maximum or rated values. This works satisfactorily for clutches and brakes in which the armature and rotor are not separated by a gap, but instead relatively rub with light contact when the clutch is "disengaged". In this sort of arrangement, slippage gradually decreases, torque gradually increases and "chatter" does not occur. Mechanical shocks on a prime mover and associated components are alleviated when a gapless-type clutch or brake is excited with a smooth ramp to produce a "soft start".
Although a "soft-start" zero-gap clutch utilizing a ramp voltage is an improvement over a clutch initially energized by a step voltage, the problem remains that the ramp-voltage engagement technique when used in connection with a gap-type clutch produces a torque coupling upon initial engagement of the armature to the rotor. This initial torque coupling is sufficient to produce a "screeching" noise resulting from slip-hold vibration as the inertia of the slower moving rotor or armature is being overcome. The "screeching" noise is similar in nature to that resulting from energization of the clutch coil by a step voltage. Also, the sudden torque-coupling engagement of the rotor and armature produced by a ramp voltage energization of the clutch coil may still cause an undesirable change in engine RPM. Therefore there remains problems of noise and sudden load changes which cannot be eliminated for a gap-type clutch energized by only a ramp voltage according to the known soft-start technique. Specifically, when the ramp voltage and its associated current and flux density are great enough to create a force which draws the armature to the rotor they are greater than what is needed to merely hold the armature and rotor in contact and to allow maximum slippage and minimum slip-hold noise (i.e., high-pitched "screech"). Therefore, there is a near instantaneous torque coupling applied to the armature-rotor interface which is less than that associated with a step-voltage energization, but nevertheless sufficient to be characterized by the same problems.
In co-pending U.S. Pat. application Ser. No. 581,108, filed Feb. 17, 1984 by Dwight Booth, now U.S. Pat. No. 4,509,091, (and assigned to the assignee of the present application) the control unit pulses the clutch coil with full voltage for a predetermined time duration known to be sufficient to draw the armature across the gap and into contact with the rotor. At the conclusion of the full voltage pulse, the clutch coil is energized by ramping up the average coil current from an initial value which is affirmatively and markedly reduced from that produced by full voltage energization of the coil. By energizing the clutch coil in this manner the torque coupling at the instant of gap closure is reduced. Specifically, the reduced average current excitation following the full-voltage pulse is chosen so as to produce approximately zero torque coupling and 100% slip at the rotor-armature interface.
The present applicant has recognized a further problem which is encountered when one attempts to utilize the Booth control apparatus with gap-type clutches or brakes which are not, for reasons of economy, constructed with the "auto-gap" feature disclosed in U.S. Pat. Nos. 2,705,058 and 2,970,681 and by which the air gap between a disengaged rotor and armature is maintained, through mechanical compensation, essentially constant despite substantial wearing down of the opposed friction surface faces. In non-compensated gap-type magnetic clutches or brakes employed in such environments as automobile accessory drives or the like, the coupling is initially manufactured such that the opposed steel surfaces of the two principal members (e.g., rotor and armature) are separated by a gap of a chosen first width (e.g., 0.021") when fully disengaged; but as the coupling is cycled thousands of times over its normal useful life, the surface wears down considerably and the gap, when the surfaces are fully disengaged, will increase to a second, larger width (e.g., 0.200").
The air gap between an armature and rotor's faces in a magnetic clutch (or brake) forms the highest reluctance portion of the magnetic flux path, and a change of only a few thousandths of an inch in gap width can materially increase the reluctance of the complete flux path whose other portions are formed by permeable steel. Thus, a given magnetic coupling possesses a main gap which becomes wider in the later stages of its useful life than in its earlier stages, and a greater m.m.f. is required in the wider gap case to create sufficient flux to draw the armature across the gap and into contact with the rotor.
It has been the practice of the industry to make the source voltage and parameters of the coil excitation circuit for such clutches create sufficient coil current and m.m.f. to close the gap and engage the cooperating members even when the gap has the widest value to be encountered near the end of the coupling's useful life. But this means that the m.m.f. will really be more than required during the major portion of the coupling's useful life.
The above-identified co-pending application discloses a control procedure for an electromagnetic coupling by which the coil of the coupling is excited at full source voltage for a predetermined time interval which corresponds to that observed as required for the gap to close when one is controlling an electromagnetic coupling of a given design and size. Normally, one would choose an observed time interval near the end of the coupling's useful life; but if that choice is followed, then control of the same coupling when it is new (and the its gap is narrowest) would result in the full excitation of the coil for a significant time span after the gap has closed and before the predetermined time interval ends. Full excitation of the coil after gap closure can thus create alternate slip-and-hold chatter, with the same objectionable noise, mechanical shock and belt screech.
Finally, applicant has observed that when chatter is alleviated by a "soft start" procedure, it is not always wholly eliminated. But applicant has discovered supplemental structure modification which further reduce the intensity of objectionable noise generated by an electromagnetic coupling's main elements when they first come into contact.